It is known that a plasma processing equipment such as semiconductor producing equipment and electronic device producing equipment, and a plasma generator such as a CO2 laser beam machine, utilize plasma that is generated by high-frequency (RF). It is also known that plasma is generated through CW (continuous wave) drive or pulse drive of a high-frequency power source (RF power source).
FIG. 24 schematically illustrates driving of plasma load, using the high-frequency power source (RF (radio frequency) power source). In FIG. 24, pulse output delivered from the high-frequency power source (RF power source) 100 is supplied to load 102, such as a plasma processing equipment and a CO2 laser beam machine, via a matching box 101, thereby generating plasma.
In generating plasma through pulse drive of the high-frequency power source (RF power source), forward wave voltage of the pulse output is intermittently supplied to the plasma load, causing a repetition of plasma on and off in the plasma load.
The pulse output according to the pulse drive of the high-frequency power source is a high-frequency (RF) output that repeats on and off at frequencies ranging from a few Hz to several hundred Hz. When the high-frequency output is switched from the on-state to the off-state, or from the off-state to the on-state, a reflected wave is generated transiently, even though the plasma is pulse driven normally. By way of example, natural oscillation of a matching box, an action of plasma ignition, or the like, may be a factor of the reflected wave occurrence, and during a period from the point when the high-frequency output is switched from the off-state to the on-state, to the point when the plasma is ignited, reflection coefficient Γ becomes approximately one (Γ≈1), and this may cause a mismatching state, leading to a total reflection state for a while. At this moment, a reflected wave is generated from the plasma load toward the high-frequency power source. At a rise of process on the plasma load side, the reflected wave power is apt to become large, typically in the state where plasma has not been ignited yet. In addition, at the time of switching from the on-state to the off-state, there may exist residual reflection until extinguishment of the high-frequency energy that is supplied during the on-state.
When the reflected wave returns to the high-frequency power source from the plasma load, a standing wave is generated, which is determined by a length of an output cable, and this may cause high voltage and large current of the reflected wave, resulting in fracture of an RF power amplifying element provided in the high-frequency power source.
In order to prevent such element fracture due to the reflected wave power, it is known that an output (forward wave power) from the high-frequency power source is drooped or reduced in response to detecting the reflected wave. Reduction of the forward wave power so as to protect the high-frequency power source against the reflected wave power leads to lowering of the voltage applied to the load, and thus, there is a problem that, depending on a recipe condition that restricts atmospheric pressure and gas types, igniting plasma may fail and disable the pulse drive.
As a configuration to ignite plasma, following prior arts are known, for instance:    (a) Configuration where an igniter device for generating discharge voltage is provided as a power source for igniting plasma, in addition to a power source that supplies drive power to the plasma in a steady state. The high-frequency power source provided with the igniter device can eliminate the need for a function of protecting a power element against the total reflected power, according to a soft-start function (Patent Document 1).
Following techniques are also known as prior arts relating to plasma ignition:    (b) Technique of reducing distance between electrodes partially, so as to induce ignition even at low power (Patent Document 2);    (c) Technique of providing a matching box connected between a high-frequency power source and a load, and according to a preset function of the matching box, displacing a matching point at a plasma ignition time when the off-state is shifted to the on-state, from the matching point during a stable power supplying time in the on-state, thereby inducing generation of ignition voltage (Patent Document 3);    (d) Technique of regulating pressure within the plasma generator to a level that induces ignition (Patent Document 4); and    (e) Technique of igniting plasma during a period when average forward wave power is increased, by increasing a duty ratio of forward wave power from zero to 100% sequentially (Patent Document 5).